Prior art treatments to reduce secondary electron emission from copper surfaces included the topical application of materials having lower emission levels than untreated copper. Examples of such prior art treatments include the application of a thin titanium carbide film to a copper substrate as disclosed in a 1977 NASA Technical Paper 1097. This publication discloses another approach to the treatment of copper in which a textured surface is created by a simultaneous sputter-application of a low sputter-yield material, such as tantalum or stainless steel, to the copper which is then exposed to ion bombardment.
The formation of a hard, nonporous polycrystalline carbon film on an ion-cleaned copper substrate by ion bombardment from an ionized carbon source gas is described in U.S. Pat. No. 3,540,989 to Webb. Another attempt to improve a multistage depressed collector electrode comprises the substitution of smooth or moderately roughened graphite for the copper electrodes as described in U.S. Pat. No. 3,549,930 to Katz. Another technique utilizes ion-textured pyrolytic graphite for copper as electrode material as set forth in U.S. Pat. No. 4,417,175 to Curren et al.
One of the major disadvantages of coating copper surfaces with materials having relatively lower secondary electron emission characteristics than copper itself, such as titanium carbide or hard, nonporous polycrystalline carbon films by these prior art methods is that the resulting emission characteristics, while lower than for untreated copper, are not reduced to their lowest attainable levels. Similarly, the seeded and ion-bombarded textured copper surfaces, while yielding secondary electron emission levels significantly lower than for untreated copper, nevertheless do not produce the lowest attainable emission level.
While the ion-textured pyrolytic graphite exhibits secondary electron emission characteristics sharply lower than those of untreated copper and the other surfaces described in the prior art, the use of this textured pyrolytic graphite also has disadvantages. More particularly, this material is porous and special, time-consuming, and expensive procedures are required to remove entrapped and surface gases before the graphite can be used in a high-vacuum, high-voltage environment such as multistage depressed collector electrodes experience. Further in order to use graphite components in complex assemblies such as multistage depressed collectors, these components must be suitably attached to support structures, preferably by brazing. Such procedures require special development for reliable graphite-to-ceramic or metal interfaces, particularly for highly anisotropic pyrolytic graphite. Because of these costly complications, manufacturers of traveling-wave tubes using multistage depressed collectors are reluctant or unwilling to adopt the use of graphite electrodes.